title: Mercury cycling in agricultural and managed wetlands of California, USA: experimental evidence of vegetation-driven changes in sediment biogeochemistry and methylmercury production.
abstract: The role of live vegetation in sediment methylmercury (MeHg) production and associated biogeochemistry was examined in three types of agricultural wetlands (domesticated or white rice, wild rice, and fallow fields) and adjacent managed natural wetlands (cattail- and bulrush or tule-dominated) in the Yolo Bypass region of California's Central Valley, USA. During the active growing season for each wetland, a vegetated:de-vegetated paired plot experiment demonstrated that the presence of live plants enhanced microbial rates of mercury methylation by 20 to 669% (median=280%) compared to de-vegetated plots. Labile carbon exudation by roots appeared to be the primary mechanism by which microbial methylation was enhanced in the presence of vegetation. Pore-water acetate (pw[Ac]) decreased significantly with de-vegetation (63 to 99%) among all wetland types, and within cropped fields, pw[Ac] was correlated with both root density (r=0.92) and microbial Hg(II) methylation (kmeth. r=0.65). Sediment biogeochemical responses to de-vegetation were inconsistent between treatments for "reactive Hg" (Hg(II)R), as were reduced sulfur and sulfate reduction rates. Sediment MeHg concentrations in vegetated plots were double those of de-vegetated plots (median=205%), due in part to enhanced microbial MeHg production in the rhizosphere, and in part to rhizoconcentration via transpiration-driven pore-water transport. Pore-water concentrations of chloride, a conservative tracer, were elevated (median=22%) in vegetated plots, suggesting that the higher concentrations of other constituents around roots may also be a function of rhizoconcentration rather than microbial activity alone. Elevated pools of amorphous iron (Fe) in vegetated plots indicate that downward redistribution of oxic surface waters through transpiration acts as a stimulant to Fe(III)-reduction through oxidation of Fe(II)pools. These data suggest that vegetation significantly affected rhizosphere biogeochemistry through organic exudation and transpiration-driven concentration of pore-water constituents and oxidation of reduced compounds. While the relative role of vegetation varied among wetland types, macrophyte activity enhanced MeHg production.
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA 94025, USA. Electronic address: lwindham@usgs.gov.
authors.forename: Lisamarie
authors.initials: L
authors.lastname: Windham-Myers
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA 94025, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Marvin-DiPasquale
authors.affiliation: U.S. Geological Survey, Fort Collins Science Center, Building 21, MS 963, Denver, CO 80225, USA.
authors.forename: Craig
authors.initials: C
authors.lastname: A Stricker
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA 94025, USA.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Agee
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA 94025, USA.
authors.forename: Le
authors.initials: L
authors.lastname: H Kieu
authors.affiliation: U.S. Geological Survey, Western Region Bureau of Regional Research, 345 Middlefield Road, MS 480, Menlo Park, CA 94025, USA.
authors.forename: Evangelos
authors.initials: E
authors.lastname: Kakouros
mesh_terms: D000383:Agriculture; D002140:California; D004784:Environmental Monitoring; D019015:Geologic Sediments / Q000737:chemistry; D008628:Mercury / Q000032:analysis; D008767:Methylmercury Compounds / Q000032:analysis*; D014874:Water Pollutants, Chemical / Q000032:analysis*; D053833:Wetlands
chemical_list: D008767:Methylmercury Compounds; D014874:Water Pollutants, Chemical; D008628:Mercury; C004925:methylmercuric chloride
keywords: Carbon; Iron; Rhizosphere; Rice; Root; Transpiration